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Organic light-emitting diodes are constantly developed technologically. An advantageous strategy for efficient work of the devices is the design of emitters exhibiting thermally activated delayed fluorescence (TADF). Efforts are invested, especially towards blue TADF fluorophores. The complex nature of the involved photophysical processes prohibits straightforward prediction of dyes with enhanced TADF potential. Design rules are knowledge-based or derived for specific cases. In the present work, we attempt to establish quantitative structure–property relationships (QSPR) between a key optical property of potential TADF emitters and a set of molecular descriptors. The goal is a statistical model providing better understanding of the molecular factors governing the excited states singlet–triplet energy splitting. We utilise multiple linear regression and principal component analysis to correlate the values of the excited singlet–triplet energy gap to various molecular characteristics. The validated models demonstrate the complexity and collective nature of TADF events and highlight the essential role of the electron-donor fragment. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescent (TADF) emitters attract interest as organic light‐emitting diode (OLED) materials with potentially 100% internal quantum efficiency (IQE). Their application in commercial displays is still hindered due to the not yet sufficient operational stability. We analyze the degradation mechanism in a TADF OLED by combining electro‐optical characterization and device simulations. We find that two trap states are being created at the HTL/EML interface during initial degradation. [ABSTRACT FROM AUTHOR]

A novel class of o‐carboranyl luminophores, 2CB‐BuDABNA (1) and 3CB‐BuDABNA (2) is reported, in which o‐carborane moieties are incorporated at the periphery of the B,N‐doped multi‐resonance thermally activated delayed fluorescence (MR‐TADF) core. Both compounds maintain the inherent local emission characteristics of their MR‐emitting core, exhibiting intense MR‐TADF with high photoluminescence quantum yields in toluene and rigid states. In contrast, the presence of the dark lowest‐energy charge transfer state, induced by cage rotation in THF, is suggested to be responsible for emission quenching in a polar solvent. Despite the different arrangement of the cage on the DABNA core, both 1 and 2 show red‐shifted emissions compared to the parent compound BuDABNA (3). By utilizing 1 as the emitter, high‐efficiency blue organic light‐emitting diodes (OLEDs) are achieved with a remarkable maximum external quantum efficiency of 25%, representing the highest reported efficiency for OLEDs employing an o‐carboranyl luminophore as the emitter. [ABSTRACT FROM AUTHOR]

Two derivatives of phenyl pyrimidine as acceptor unit and triphenylamino or 4,4′-dimethoxytriphenylamino donor groups were designed and synthesized as emitters for organic light-emitting diodes (OLEDs) aiming to utilize triplet excitons in the electroluminescence. Thermogravimetric analysis revealed high thermal stability of the compounds with 5% weight loss temperatures of 397 and 438 °C. The theoretical estimations and photophysical data show the contributions of local excited and charge transfer states into emission. The addition of the methoxy groups led to the significant improvement of hole-transporting properties and the bathochromic shift of the emission from blue to green-blue spectral diapason. It is shown that mixing of the compounds with the organic host results in facilitation of the delayed emission. The singlet–triplet energy splitting was found to be too big for the thermally activated delayed fluorescence. No thermal activation of the long-lived emission was detected. No experimental evidence for triplet–triplet annihilation and room temperature phosphorescence were detected making the hot exciton mechanism the most probable one. The OLEDs based on the compounds reached the maximum external quantum efficiency of up to 10.6%. [ABSTRACT FROM AUTHOR]

This study proposes a novel approach to develop highly efficient, narrow‐emitting violet materials based on boron and oxygen polycyclic aromatic hydrocarbon multiple resonance structure. Herein, B‐2OCz is developed by fusing indole with a 5,9‐dioxa‐13bboranaphtho[3,2,1‐de]anthracene (DOBNA) core to enhance its thermally activated delayed fluorescence (TADF) properties and molecular rigidity. On the other hand, the B‐2OCz‐Si is decorated with a bulky tetraphenylsilyl substituent. B‐2OCz‐Si exhibits exceptional features such as violet emission at 397 nm, a very small full width at half maximum of 16 nm, and 82% of photoluminescence quantum yield. The B‐2OCz‐Si devices achieve a high external quantum efficiency of over 15%, violet emission with a peak wavelength of 423 nm, and color coordinates of (0.156, 0.037). Furthermore, the B‐2OCz‐Si is used as an electron transport type host material for phosphorescent organic light‐emitting diodes (PhOLEDs), based on its high triplet energy and TADF properties. As compared to the conventional triazine based host materials, these newly developed DOBNA‐based materials display superior device lifetime performance. All these potential aspects corroborate that this new class of DOBNA‐based materials can work as a promising host material for PhOLEDs and violet‐emitting fluorescent devices. [ABSTRACT FROM AUTHOR]

This research focuses on a theoretical study of diphenylsulfone-based emitters for thermally activated delayed fluorescence (TADF). The acceptor moiety employed in this study is sulfonyldibenzene, while the donor moiety is 9H-carbazole. The effects of methoxy, thiophene, pyridine, and pyrazine adding on the donor moiety were investigated, resulting in the design of six distinct molecules. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) methods, utilizing the B3LYP functional with 6-311G(d,p) basis sets, were used to assess the properties of these compounds in their singlet, doublet, and triplet states. The calculations encompassed various aspects, including ground state molecular geometry, electronic properties, dipole moments, polarizabilities, frontier orbital energies, density of states, UV-Vis spectra, and IR spectra. The investigation revealed that adding the donor moiety with rings, particularly thiophene and pyridine, resulted in improved properties, including molecular geometry. Furthermore, increasing the D-A dihedral angle was found to increase the energy gap, optical gap, dipole moments, and polarizabilities, while reducing the ∆EST and the overlap between the HOMO and LUMO. Hole-electron analysis demonstrated that all the compounds exhibited localized excitation characteristics, with the electrons being photoinduced locally. These compounds exhibited strong blue emissions, with maximum wavelengths ranging from 387.462 to 458.286 nm in the singlet-triplet states. Moreover, the designed compounds exhibited coverage of the entire visible region in their UV-Vis spectra when in the doublet state. Based on these findings, these compounds show promise as TADF emitters in organic light-emitting diodes (OLEDs), providing improvements in efficiency and properties. [ABSTRACT FROM AUTHOR]

Efficient ultraviolet (UV) electroluminescent materials remain a great challenge, since short peak wavelength <400 nm and narrow full width at half maximum (FWHM) <50 nm are simultaneously required. In this sense, multi‐resonance (MR) thermally activated delayed fluorescence (TADF) emitters featuring narrow‐band emissions hold the promise for UV applications. Herein, a novel MR‐TADF skeleton featuring carbazole‐phosphine oxide (P=O) fused aromatics is developed to construct the first two UV MR emitters named CzP2PO and tBCzP2PO. In addition to synergistic resonance effects of P=O and N atom, sp3‐hybrid P atom renders the curved polycyclic planes of CzP2PO and tBCzP2PO, giving rise to their narrowband UV emissions with peak wavelengths <390 nm and FWHM<35 nm. Besides configuration quasi‐planarization for radiation enhancement and quenching suppression, P=O moiety further enhances singlet‐triplet coupling to facilitate reverse intersystem crossing, resulting in the state‐of‐the‐art photoluminescence quantum yield of 62 % in tBCzP2PO doped films. As consequence, tBCzP2PO endowed its UV organic light‐emitting diodes with the peak at 382 nm and FWHM of 32 nm, and especially the record‐high external quantum efficiency (EQE) of 15.1 % among all kinds of UV devices. Our results demonstrate great potential of P=O based MR emitters in practical applications including optoelectronics, biology and medicine science. [ABSTRACT FROM AUTHOR]

The investigation of organic light‐emitting diodes (OLEDs) and organic laser devices with thermally activated delayed fluorescence (TADF) molecules is emerging due to the potential of harnessing triplets. In this work, a boron/nitrogen multiple‐resonance TADF polycyclic framework fusing carbazole units (CzBNPh) was proposed. CzBNPh exhibited a narrowband emission (<30 nm), a unity photoluminescence quantum yield, and a fast radiative rate. Consequently, CzBNPh demonstrated a low distributed feedback (DFB) lasing threshold of 0.68 μJ cm−2. Furthermore, the stimulated emission zone of CzBNPh was effectively separated from its singlet and triplet absorption, thereby minimizing the singlet‐triplet annihilation under long‐pulsed excitation ranging from 20 μs to 2.5 ms. Significantly, the enhanced rigid molecular conformation, thermal stability, and photo‐stability resulted in improved lasing and electroluminescence stability compared to that of 5,9‐diphenyl‐5,9‐diaza‐13b‐boranaphtho[3,2,1‐de]anthracene (DABNA)‐core. These findings indicate the potential of CzBN‐core as a promising framework for achieving long‐pulsed wave and electrically‐pumped lasing in the future. [ABSTRACT FROM AUTHOR]

A crystalline host matrix (CHM) with embedded nanoaggregates (NA) is a powerful strategy to develop high‐performance crystalline OLEDs that can combine the advantages of both crystalline and high‐exciton‐utilization‐efficiency light emitting materials. Here a CHM‐NA structure blue‐emission crystalline OLED is developed, which employs a thermally activated delayed fluorescence (TADF) material. The CHM‐TADFNA OLED is proved to efficiently manage exciton distribution by engineering nanoaggregates. The device exhibits a remarkable EQE up to 10.4% and achieves the characteristics of rapid turn‐on, and fast ramping of luminance and current density, resulting in an enhanced photon output and a lower series‐resistance Joule‐heat loss ratio, indicating a competitive pathway to future blue‐emission OLEDs. [ABSTRACT FROM AUTHOR]

B‐ and N‐embedded multiple resonance (MR) type thermally activated delayed fluorescence (TADF) emitters usually suffer from slow reverse intersystem crossing (RISC) process and aggregation‐caused emission quenching. Here, we report the design of a sandwich structure by placing the B−N MR core between two electron‐donating moieties, inducing through‐space charge transfer (TSCT) states. The proper adjusting of the energy levels brings about a 10‐fold higher RISC rate in comparison with the parent B−N molecule. In the meantime, a high photoluminescence quantum yield of 91 % and a good color purity were maintained. Organic light‐emitting diodes based on the new MR emitter achieved a maximum external quantum efficiency of 31.7 % and small roll‐offs at high brightness. High device efficiencies were also obtained for a wide range of doping concentrations of up to 20 wt % thanks to the steric shielding of the B−N core. A good operational stability with LT95 of 85.2 h has also been revealed. The dual steric and electronic effects resulting from the introduction of a TSCT state offer an effective molecular design to address the critical challenges of MR‐TADF emitters. [ABSTRACT FROM AUTHOR]

We report on organoboron complexes characterized by very small energy gaps (ΔEST) between their singlet and triplet states, which allow for highly efficient harvesting of triplet excitons into singlet states for working as thermally activated delayed fluorescence (TADF) devices. Energy gaps ranging between 0.01 and 0.06 eV with dihedral angles of ca. 90° were registered. The spin–orbit couplings between the lowest excited S1 and T1 states yielded reversed intersystem crossing rate constants (KRISC) of an average of 105 s−1. This setup accomplished radiative decay rates of ca. 106 s−1, indicating highly potent electroluminescent devices, and hence, being suitable for application as organic light-emitting diodes. [ABSTRACT FROM AUTHOR]

Utilizing purely organic materials with relatively highly emissive characteristics for designing organic light-emitting diodes (OLEDs) is fascinating since employing rare metals is unessential, hence, enormous research activities have been conducted in this field. Recently, additional efforts were devoted to designing special emitting materials demonstrating thermally activated delayed fluorescence (TADF). Molecular structures were ideally designed in a specific way using an electron-deficient molecule as an acceptor (A) and an electron-rich molecule as a donor (D) in order to minimize the overlapping between the highest occupied molecular orbital (HOMO) of the donor (D) and the lowest unoccupied molecular orbital (LUMO) of the acceptor (A). TADF requires a small band gap between the singlet excited and triplet excited states (ΔEST), usually less than (c.a 0.1 eV) of the luminescent material. This makes it possible to thermally harvest the excitons from the triplet excited state (T1) to the singlet excited state (S1) by a process known as reverse intersystem crossing (RISC) and then to harvest all the excitons for the fluorescence process. Thus, the design of the TADF molecular structure needs both an electron-rich segment as a donor and an electro-deficient segment as an acceptor. Up to the present, electron-deficient moieties (EDM) have been studied less than electron rich moieties (ERM) during the last few years. This review is highlighting the recent advances regarding the specific molecular designs of TADF emission materials for OLED applications, particularly, focusing on (dibenzothiophene-S,S-dioxide) as electron deficient molecule, which exhibits a strong electron-deficient unit and high ability of photo- and electroluminescence features. [ABSTRACT FROM AUTHOR]

Designing multi‐resonance (MR) emitters that can simultaneously achieve narrowband emission and suppressed intermolecular interactions is challenging for realizing high color purity and stable blue organic light‐emitting diodes (OLEDs). Herein, a sterically shielded yet extremely rigid emitter based on a triptycene‐fused B,N core (Tp‐DABNA) is proposed to address the issue. Tp‐DABNA exhibits intense deep blue emissions with a narrow full width at half maximum (FWHM) and a high horizontal transition dipole ratio, superior to the well‐known bulky emitter, t‐DABNA. The rigid MR skeleton of Tp‐DABNA suppresses structural relaxation in the excited state, with reduced contributions from the medium‐ and high‐frequency vibrational modes to spectral broadening. The hyperfluorescence (HF) film composed of a sensitizer and Tp‐DABNA shows reduced Dexter energy transfer compared to those of t‐DABNA and DABNA‐1. Notably, deep blue TADF‐OLEDs with the Tp‐DABNA emitter display higher external quantum efficiencies (EQEmax=24.8 %) and narrower FWHMs (≤26 nm) than t‐DABNA‐based OLEDs (EQEmax=19.8 %). The HF‐OLEDs based on the Tp‐DABNA emitter further demonstrate improved performance with an EQEmax of 28.7 % and mitigated efficiency roll‐offs. [ABSTRACT FROM AUTHOR]

Developing double boron‐based emitters with extremely narrow band spectrum and high efficiency in organic light‐emitting diodes (OLEDs) is crucial and challenging. Herein, we report two materials, NO‐DBMR and Cz‐DBMR, hinge on polycyclic heteraborin skeletons based on role‐play of the highest occupied molecular orbital (HOMO) energy levels. The NO‐DBMR contains an oxygen atom, whereas the Cz‐DBMR has a carbazole core in the double boron‐embedded ν‐DABNA structure. The synthesized materials resulted in an unsymmetrical pattern for NO‐DBMR and surprisingly a symmetrical pattern for Cz‐DBMR. Consequently, both materials showed extremely narrow full width at half maximum (FWHM) of 14 nm in hypsochromic (pure blue) and bathochromic (Bluish green) shifted emission without losing their high color fidelity. Furthermore, both materials show high photoluminescence quantum yield (PLQY) of over 82 %, and an extremely small singlet‐triplet energy gap (ΔEST) of 0.04 eV, resulting in high reverse intersystem crossing process (kRISC) of 105 s−1. Due to the efficient thermally activated delayed fluorescence (TADF) characteristics, the fabricated OLEDs based on these heteraborins manifested maximum external quantum efficiency (EQEmax) of 33.7 and 29.8 % for NO‐DBMR and Cz‐DBMR, respectively. This is the first work reported with this type of strategy for achieving an extremely narrow emission spectrum in hypsochromic and bathochromic shifted emissions with a similar molecular skeleton. [ABSTRACT FROM AUTHOR]

As a promising technology toward high‐efficiency organic light‐emitting diode (OLED), thermally activated sensitized fluorescence (TASF) has encountered major obstacles toward achieving sufficient operational lifetime when it comes to deep blue emission that is desirable for display applications. We have systematically investigated various factors affecting TASF OLED stability and successfully improved device lifetime by using a thermally activated delayed fluorescence (TADF) sensitizer with a fast reverse intersystem crossing rate (kRISC). [ABSTRACT FROM AUTHOR]

Two narrow emitting emitters were developed via a carbon/nitrogen (C/N) fusion strategy. The fusion‐induced rigid structures induced an MR effect similar to the boron frameworks. Both compounds offer narrow emission, very small Stocks shift, and high photoluminescence quantum yield. The fluorescence devices fabricated using m‐FLDID offer maximum efficiency (EQEmax) of above 5%, pure violet emission at 407 nm, and FWHM of 17 nm. Whereas the p‐ FLDID‐based thermally activated delayed fluorescence (TADF) devices showed EQEmax of above 25%, deep‐blue emission at 452 nm, and FWHM of 23 nm. [ABSTRACT FROM AUTHOR]

Organic light‐emitting diodes (OLEDs) are a widespread commercial display technology but they are still subject of ongoing research with the aim to expand their application ranges through stack and material design. Operational stability is one of the most important key properties that are steadily being improved. The long lifetimes of several thousands of hours of state‐of‐the‐art OLED structures at standard operating conditions demand for accelerated lifetime tests (ALTs) and the use of appropriate scaling laws for elevated temperatures and current densities. Here we present ALTs for a blue thermally activated delayed fluorescent (TADF) OLED and demonstrate the successful determination of the scaling parameters. Moreover, we study the microscopic degradation mechanisms using in‐situ electrical characterization methods. [ABSTRACT FROM AUTHOR]

[Display omitted] Although OLEDs are widely employed nowadays for display technology devices, their application for room-lighting illumination remains a challenge due to the cost-effectiveness issues, mainly related to device fabrication. In this sense, the present study investigates the optimization of blue-emitting TADF (DMOC-DPS) and yellow-emitting TADF (TXO-TPA) compounds in solution-processed OLEDs to achieve efficient white light emission in a two-organic layer device. Four different host materials were studied, aiming to balance the charge mobility of holes and electrons. The host materials used include (in %wt.) a 1:1 mixture of mCP and DPEPO (HOST1), a 3:2 mixture of PVK and DPEPO (HOST2), a 3:2 mixture of PVK and mCP (HOST3), and a 3:2 mixture of PVK and butyl-PBD (HOST4). The experimental results obtained from the solution-processed OLEDs indicate that DMOC-DPS is predominantly a hole transport material, and hosts with predominantly n-type character, such as HOST1 and HOST4 , resulting in the most efficient white-OLEDs by the most balanced charge mobility. With structure optimization, WOLEDs achieved 6.43 % EQE with a brightness of 2621 cd/m2 (not integrated) and 6.06 % EQE with a brightness of 1986 cd/m2 for HOST4 and HOST1 , respectively. The emission characteristics were influenced by host materials characteristics, with blue and yellow emissions being fine-tuned to produce complementary colors. This study highlights the critical role of charge mobility balance in the emissive layer and demonstrates the potential of independently optimizing blue and yellow TADF components for high-performance WOLEDs suitable for indoor lighting applications. [Display omitted] • New approaches for solution-deposited WOLEDs based on complimentary color TADF emitters. • Electrical transport/recombination optimization in TADF host: guest WOLED deposited by solution-process techniques. • OLEDs simulation towards simplifyed/efficent device structure development. [ABSTRACT FROM AUTHOR]

[Display omitted] • Role of electron acceptor substitution such as −CF 3 and –CN in PXZOXD were studied. • Photophysical analysis of the acceptor strength increment in PXZOXD is securitized. • Molecular photophysical changes delved with DFT and TDDFT studies. • OLEDs fabricated with –CN added PXZOXD showed greater performances with low ΔE ST. Herein, we reported the effect of electron withdrawing units, such as trifluoromethyl (CF 3) and cyanide (CN), substitution on a thermally activated delayed fluorescent (TADF) molecule (10-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)-10 H -phenoxazine (PXZ-OXD)). The addition of electron withdrawing units has increased the acceptor strength and reduced the interaction between PXZ and OXD, thus reducing the gap between the singlet and triplet states (ΔE ST) of excitons. To understand the variation in the acceptor strength of PXZ-OXD as a function of its molecular structure and density of states, density functional theory (DFT) and time-dependent DFT (TDDFT) were conducted. Transition density matrix investigations revealed that the addition of −CF 3 and –CN to PXZ-OXD triggers CT excitons, while inducing lower ΔE ST values. Particularly, the lowest ΔE ST (0.05 eV) with a notable red shift in the emission spectrum was observed with 4′-CF 3 PXZOXD. The delayed component lifetime of PXZOXD is found to be reduced after the substitution of −CNwhm and −CF 3. Despite the weak charge transfer transitions, the substitution of conjugative –CN group is found to be beneficial in improving the HOMO-LUMO overlap with a moderate decrease in reverse intersystem crossing (K RISC), which attained enhancement in the photoluminescent quantum yield. Additionally, the substitution of the above electron withdrawing units on PXZ-OXD yielded a red shift in the electroluminescence spectrum. Furthermore, the external quantum efficiency (at 100 cd/m2, i.e., EQE 100) of the 4′-CNPXZOXD-based organic light-emitting diode is found to improve by 21.13 % against the PXZOXD (16.9 %). [ABSTRACT FROM AUTHOR]

Doping thermally activated delayed fluorescence (TADF) materials with high exciton utilization into crystalline hosts with high carrier mobility is an effective approach for developing novel OLEDs. This approach harnesses the strengths of both materials to realize high-performance blue light-emitting crystalline organic light-emitting diodes (C-OLEDs). Nevertheless, the high triplet energy levels of blue emitting TADF materials may facilitate the outflow of triplet excitons through Dexter energy transfer to the lower energy levels within the crystalline host, thus leading to efficiency losses in the device. In this study, we present a pioneering strategy designed to improve the exciton utilization efficiency of TADF materials in C-OLED by leveraging the up-conversion capability of TTA materials to reclaim triplet excitons. With a well-designed energy level structure, this device achieves a maximum EQE of 5.6 % and a low turn-on voltage of 2.7 V. The benefits of the crystalline host allowed for fast turn-on, and a rapid increase in brightness and current density, leading to significantly improved blue photon output and a lower series resistance Joule heat loss ratio. This work introduces a novel approach to employ TADF materials in crystalline hosts and manage excitons within the emissive layer of devices, aiming to develop high-performance C-OLEDs. [Display omitted] 1. Thermally activated delayed fluorescence (TADF) materials have been introduced as dopants in crystalline organic light-emitting diodes (C-OLEDs) for the first time. This significantly expands the range of high-exciton-utilization dopants in the emitting layer of C-OLED, providing an important foundation for achieving high-performance OLEDs. 2. The TTA sensitizer MADN was introduced into the emissive layer. The TTA sensitizer can capture triplet excitons that leak to the host 2FPPICz and further enhance exciton utilization through the TTA process. 3. Besides having a low turn-on voltage of 2.7 V (@ 1 cd m-2), the device exhibits rapid current density and brightness escalation with increasing driving voltage. The brightness of the device can be up to 1000 cd m−2 at a voltage of 3.9 V. The emitted photon quantity of the CHM-TTAS-TADF structure outperformed all the performance of reported amorphous blue OLEDs which have high EQE and similar emitting color (The CIEy is approximately 0.10), which demonstrating its superiority in the development of next-generation OLEDs. [ABSTRACT FROM AUTHOR]

Interface exciplex represents a promising host material for organic light‐emitting diodes (OLEDs) with barrier‐free charge injection and highly confined recombination region. However, the efficiency of radiative recombination in pristine exciplex is usually low and needs to be improved by doping various emitters. In this study, the interface exciplex OLEDs doped with fluorescence, phosphorescence, and thermally activated delayed fluorescence (TADF) emitters is fabricated to investigate the relationship between their excited‐state properties and electroluminescence efficiencies. A maximum external quantum efficiency of 20% is achieved in interface exciplex OLEDs doped with TADF emitter, which corresponds to nearly 100% exciton utilization and is superior to those of fluorescence and phosphorescence emitters. Furthermore, optical spectroscopy and magneto‐electroluminescence method are used to study the advantages of TADF emitter in interface exciplex host. The large dipole of TADF emitter is beneficial for harvesting energy from the charge‐transfer state at the interface, and its reverse intersystem crossing avoids the accumulation of triplet excitons that leads to triplet‐triplet annihilation in interface exciplex OLEDs. These results demonstrate that the photophysical process needs to be carefully considered in designing high‐performance emitters for exciplex host materials, and it may bring in‐depth understanding on improving exciton utilization and electroluminescence efficiency in interface exciplex OLEDs. [ABSTRACT FROM AUTHOR]

Thermally activated delayed fluorescence (TADF) materials, which can harvest all excitons and emit light without the use of noble metals, are an appealing class of functional materials emerging as next-generation organic electroluminescent materials. Triplet excitons can be upconverted to the singlet state with the aid of ambient thermal energy under the reverse inter-system crossing owing to the small singlet–triplet splitting energy (ΔEST). This results from a specific molecular design consisting of minimal overlap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital, due to the spatial separation of the electron-donating and electron-releasing part. When a well-designed device structure is applied, high-performance blue-emitting TADF organic light-emitting diodes can be realized with an appropriate molecular design. Unlike the previous literature that has reviewed general blue-emitting TADF materials, in this paper, we focus on materials other than pure organic molecules with twist D-π-A structures, including multi-resonance TADF, through-space charge transfer TADF, and metal-TADF materials. Cutting-edge molecules with extremely small and even negative ΔEST values are also introduced as candidates for next-generation TADF materials. In addition, OLED structures used to exploit the merits of the abovementioned TADF emitters are also described in this review. [ABSTRACT FROM AUTHOR]

The practical use of luminescent mononuclear gold(I) complexes as optoelectronic materials has been limited by their inferior stability. Herein we demonstrate a strategy to improve the stability of gold(I) complexes which display thermally activated delayed fluorescence (TADF). A highly rigid and groove‐like σ‐donating aryl ligand has been used to form dual Au⋅⋅⋅H−C hydrogen bonds. The secondary metal‐ligand interactions have been authenticated by single‐crystal structure, NMR spectroscopy and theoretical simulations. The TADF AuI complex exhibits appealing emission properties (photoluminescence quantum yield=76 %; delayed fluorescence lifetime=1.2 μs) and much improved thermal and photo‐stability. Vacuum‐deposited organic light‐emitting diodes (OLEDs) show promising electroluminescence with a maximum external quantum efficiency (EQE) over 23 % and negligible efficiency roll‐off even at 10 000 cd m−2. An estimated LT50 longer than 77 000 h with initial luminance of 100 cd m−2 reveals good operational stability. This work suggests a way for design of stable luminescent gold(I) complexes. [ABSTRACT FROM AUTHOR]

[Display omitted] • New triphenylpyrimidines with different donors synthesized. • Experimental signs of SSLE, TADF, RTP and TTA detected for one compound. • Mechanochromism predetermined by conformational heterogeneity. • Efficient OLED and oxygen sensor application operating utilizing triplet states. Three derivatives of fluorinated triphenylpyrimidine with the attached carbazole, phenothiazine, or acridan donor moieties are synthesized by Buchwald-Hartwig reactions. The impact of the donor units on emissive and other properties of the compounds is reported. The compounds exhibit excellent thermal stability, competitive photophysical phenomena such as room temperature phosphorescence (RTP) appearing when compounds are molecularly dispersed in the rigid polymer matrix and thermally activated delayed fluorescence (TADF). The compounds with carbazole and phenothiazine donor moieties show the manifestation of triplet–triplet annihilation in the electroluminescence when used as emitters in organic light-emitting diodes (OLEDs). The phenothiazine-containing compound exhibit dual photoluminescence with the blue-shifted peak corresponding to the quasi-axial conformer and a red-shifted peak to the quasi-equatorial conformer. This derivative shows reversible shifts of emission spectra exceeding 100 nm due to the stable (at least 4 cycles) mechanochromic luminescence under the application of external stimuli. After grinding the emission intensity maximum is observed at 555 nm, after fuming at. ca 448 nm and after melting at 555 nm. The photoluminescence shifts and ON/OFF delayed fluorescence of the phenothiazine-based emitter occur due to the alteration between the crystalline and amorphous states. Optimization of the device structure allows to control the charge balance resulting in external quantum efficiency of up to 5.7 % observed for the OLED based on the phenothiazine-based emitter. This compound also shows the biggest response to the presence of oxygen acting as the quencher of triplet excited energy. The film of the compound doped in rigid Zeonex shows an 8.4-fold increase in emission intensity after evacuation. The optical sensor fabricated using the derivative of fluorinated triphenylpyrimidine and phenothiazine is characterized by the Stern-Volmer constant 1.37 × 10-4 ppm−1. [ABSTRACT FROM AUTHOR]

This study presents the design, synthesis, and comprehensive theoretical and photophysical analysis of two new D 3 -A type thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diode (OLED) applications. Utilizing a triarylboron core as the electron-accepting group and phenothiazine (PTZ) or phenoselenazine (PSZ) as electron-donating units, the molecules BTP-S and BTP-Se were developed. The D 3 -A structure supports the separation of frontier molecular orbitals (FMOs), leading to minimized singlet-triplet energy gaps (ΔE ST), which are crucial for the TADF mechanism. Density functional theory (DFT) calculations presented that BTP-S and BTP-Se exhibit band gaps (E g) of 2.52 and 3.23 eV, respectively, with BTP-S showing an ΔE ST value as low as 0.007 eV for the S 1 -T 1 transition at the lowest energy conformation. Photophysical studies revealed high photoluminescence quantum yields (PLQYs) for both compounds, with BTP-S achieving up to 85 % in mCP films and BTP-Se up to 59 %. In vacuum-processed OLEDs, BTP-S achieved a maximum external quantum efficiency (EQE) of 25.3 %, a current efficiency (ηc) of 195.8 cd/A, and a maximum luminance (Lmax) of 17356 cd/m2, while BTP-Se reached an EQE of 7.5 %, an ηc of 132.19 cd/A, and an Lmax of 16826 cd/m2 likely limited by the contributions of a folded-donor conformer enabled by the Se substitution. These findings underscore the impact of donor unit selection and conformation on the TADF characteristics, and provide valuable insights for designing high-performance OLED materials. [Display omitted] • Two novel D3-A type TADF emitters, BTP-S and BTP-Se, were designed and synthesized. • Both theoretical and photophysical results are consistent with a thermally activated mechanism of delayed fluorescence. • High photoluminescence quantum yields: BTP-S up to 85 %, BTP-Se up to 59 % in mCP films. • BTP-Se reached EQE of 7.5 %, ηc of 132.19 cd/A, and Lmax of 16826 cd/m2. • BTP-S achieved EQE of 25.3 %, ηc of 195.8 cd/A, and L max of 17356 cd/m2. [ABSTRACT FROM AUTHOR]

[Display omitted] • Isomeric pairs of 2,3- and 3,4-triptycene (Tp) fused multi-resonance TADF emitters were prepared. • The 3,4-Tp-fused emitter exhibits 6-fold faster reverse intersystem crossing (RISC) but shows PL spectral broadening compared to the 2,3-isomer. • Stronger SOC between the S 1 and T 2 excited states, as well as smaller activation energy of RISC, is observed for the 3,4-Tp-fused emitter. • Blue TADF-OLEDs based on each isomeric emitter similarly achieve a high maximum EQE of ∼ 26 %. • The devices with the 3,4-isomer exhibit significantly reduced efficiency roll-offs compared to the 2,3-isomer. Concurrently achieving high efficiency, narrowband emission, and low efficiency roll-off remains a challenging task for realizing stable blue organic light-emitting diodes (OLEDs) with high color purity. To tackle this issue, herein, two isomeric pairs of B,N-doped multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters, namely, 2,3-Tp-BuDABNA-R and 3,4-Tp-BuDABNA-R (R=H, 3,5-Xyl), featuring triptycene (Tp) moieties fused into the 2,3- and 3,4-positions of the B,N core, respectively, are presented. Both isomeric forms exhibit blue emissions with near-unity photoluminescence (PL) quantum yields in their doped host films. Notably, the 3,4-Tp-fused emitter shows 6-fold faster reverse intersystem crossing (RISC) compared to the 2,3-Tp-fused emitter, albeit with PL spectral broadening. Along with a smaller activation energy of RISC, the stronger spin–orbit coupling between the singlet (S 1) and triplet (T 2) excited states of the 3,4-Tp-fused emitter is suggested to facilitate the RISC process. Furthermore, a high-frequency vibration in the Tp-fused benzene ring, accompanied by a large reorganization energy, is found to mainly contribute to the spectral broadening of the 3,4-Tp-fused emitter. Blue TADF-OLEDs based on each isomeric emitter similarly achieve a high maximum external quantum efficiency of ∼ 26%. Remarkably, the devices with the 3,4-isomer exhibit significantly reduced efficiency roll-offs compared to the 2,3-isomer, demonstrating that isomer engineering of MR emitters can have a distinct impact on controlling device performance. [ABSTRACT FROM AUTHOR]

Exploring through-space charge transfer (TSCT) excited state for the design of thermally activated delayed fluorescence (TADF) emitters has been receiving increasing interest for organic light-emitting diodes (OLEDs). In this work, we developed two TSCT-TADF molecules, namely 2DPXZ-QX and 2DPXZ-DFQX , which have sandwich-type donor-acceptor-donor (D-A-D) structures supported by two carbazole bridges. Dibenzo[a,c]phenazine (QX) and its fluorinated derivative (DFQX) were used as the acceptors and O-bridged triphenylamine (DPXZ) was used as the donor. The two donors and acceptor in each molecule are aligned in a face-to-face orientation and thus result in the presence of intramolecular π–π interactions, as revealed by their single crystal X-ray structures. The emission maxima (λ PL) of 2DPXZ-QX and 2DPXZ-DFQX in doped 1,3-bis(N-carbazolyl)benzene films are 595 and 600 nm with photoluminescence quantum yields of 67 % and 54 %, respectively. The delayed fluorescence lifetimes are 8.8 and 6.7 μs. OLEDs based on the new sandwich-type emitters show maximum external quantum efficiencies of up to 19.1 %. [Display omitted] • Cofacial donor-acceptor-donor for through-space charge transfer excited states. • Balanced radiative decay and reverse intersystem crossing rate constants. • Improved thermally activated delayed fluorescence owing to strong π-π interactions. • Red organic light-emitting diodes with high external quantum efficiencies. [ABSTRACT FROM AUTHOR]

This work describes the synthesis and characterisation of new tetrahedral boron complexes, incorporating bromine- or iodine-substituted 8-quinolinolato chelate chromophores connected to 9-borafluoren-9-yl or diphenylboron orthogonal fragments. The molecular features and photophysical properties of these complexes are analysed in both solution and solid state. Steady-state photophysical studies reveal photoluminescence quantum yields (Φ f) ranging from 0.02 to 0.15 and prompt fluorescence (PF) lifetimes (τ f) between 2 and 16 ns. Time-resolved photophysical experiments show the presence of delayed fluorescence (DF) and phosphorescence at both 77 K and room temperature. The DF intensity increases with a rise in temperature. This variation is ascribed to an enhancement in the intersystem crossing (ISC) process promoted by the bromine or iodine heavy-atom effect. Investigations into the dependence of DF intensity relative to the excitation dose indicate emissions stemming either from Triplet-Triplet Annihilation (TTA), Thermally Activated Delayed Fluorescence (TADF), or a combination of these competing mechanisms. The effect is related to the size and number of heavy-atom substituents in each boron complex. A study of the DF emission intensity as a function of the excitation dose reveals that diiodo-substituted 8-quinolinolato boron complexes, whether rigid or flexible, display TADF emission. Rigid 5,7-dibromo- and 5-chloro-7-iodo-substituted 8-quinolinolato complexes exhibit a combined TADF-TTA mechanism, whereas the other complexes predominantly demonstrate pure TTA emission. DFT and TDDFT calculations showed that the ground state structures reproduced the experimental geometries and only small increases in bond lengths were observed in the excited state geometries. The low energy absorption bands displayed mainly intra-ligand π→π* (8-quinolinato) character. The fluorescence emission energies were well reproduced, while the singlet-triplet energy gaps were relatively high and spin-orbit coupling was relevant for complexes with heavy-atoms. Ultimately, organic light-emitting diodes (OLEDs) are fabricated using the most luminescent boron complexes. The best OLED is obtained when using complex 3a , which displays green electroluminescence (EL) (λ EL = 502 nm) with maximum external quantum efficiency (EQE max) of 2.5% and maximum luminance (L max) of 2200 cd m−2. • New boron complexes with Br- or I-substituted 8-quinolinolato ligands were prepared. • DFT/TDDFT calculations revealed 8-quinolinolato intra-ligand electronic transitions. • The new boron complexes exhibited Delayed Fluorescence (DF). • The DF arises from either TTA, TADF or mixed TTA-TADF processes. • Selected boron complexes were tested as dopants in OLED devices. [ABSTRACT FROM AUTHOR]

There is increasing interest in thermally activated delayed fluorescence (TADF) in materials, and to understand its mechanism in the excited state dynamics. Recent challenges include color purity, efficient deep‐blue emission, fast exciton decay lifetimes, high reverse intersystem crossing rates (kRISC), low‐efficiency roll‐off in organic light‐emitting diodes (OLEDs), and long device lifetimes. Here, a series of compounds having benzonitrile and carbazole rings are examined, that provide a detailed understanding of the excited states, and a guideline for high‐performance TADF. A dense alignment of the excited states with several different characters within a small energy range results in high kRISC of >2 × 106 s−1, while maintaining radiative rate constants (kr) >107 s−1. OLEDs based on the optimum compound exhibit a low‐efficiency roll‐off and a CIEy (y color coordinate of Commission Internationale de l'Éclairage) <0.4. TADF‐assisted fluorescence (TAF) OLED exhibits a maximum external quantum efficiency of 22.4% with CIE coordinates (0.13,0.15). This work also provides insights for device engineering and molecular designs. [ABSTRACT FROM AUTHOR]

There are many reports on thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) with the external quantum efficiency achieving 30%. We investigate the electrical conduction of 4CzIPN-based TADF OLEDs depending on doping concentration of 0, 10, 20 or 35 mol%. As the doping concentration of 4CzIPN is increased, the electrons injected into the 4CzIPN sites are easily hopping between the 4CzIPN sites. The drive voltage decreases but the power efficiency is not remarkably improved. Since the red-shift of EL spectra at a constant current density is observed with doping concentration, the main carrier recombination zone is found to move with doping concentration. [ABSTRACT FROM AUTHOR]

The employment of thermally activated delayed fluorescence (TADF) emitters is one of the most promising ways to realize the external quantum efficiency (EQE) of over 25% for organic light-emitting diodes (OLEDs). In addition, the TADF emitter based on oxygen-bridged boron (BO) fragment can maintain blue emission with high color purity. Herein, we constructed two blue TADF emitters, 3TBO and 5TBO, for OLEDs application. Both emitters consist of three donors linked at the oxygen-bridged boron acceptor. OLED devices based on 3TBO and 5TBO exhibited both high excellent device efficiency and high color purity with a maximum EQE; full-width at half-maximum (FWHM); and CIE coordinates of 17.3%, 47 nm, (0.120, 0.294), and 26.2%, 57 nm, (0.125, 0.275), respectively. [ABSTRACT FROM AUTHOR]

An electroplex is an excited state charge transfer complex formed between a donor molecule and an acceptor molecule upon electrical excitation. Since the spatial overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of electroplex is small likewise exciplex, the energy difference between the singlet and the triplet charge transfer (1CT and 3CT) states can be very small, which facilitate the conversion of triplet excitons into singlet excitons by the assistance of thermal energy and the emission as delayed fluorescence. However, in contrast to exciplex, the detailed analysis of the reverse intersystem crossing (RISC) from 3CT to 1CT and the exciton‐related annihilation processes in electroplex OLEDs have been rarely reported. In this talk, the magnetic field effects (MFEs) on the electroplex host using 2,2'‐di(9H‐carbazol‐9‐yl)‐1,1'‐biphenyl (oCBP) and 9‐(4,6‐bis(3‐(triphenylsilyl)phenyl)‐1,3,5‐triazin‐2‐yl)‐9H‐carbazole (DSiCzTrz) were investigated. Temperature‐dependent MFE measurements revealed that RISC channel co‐exists with the intersystem crossing (ISC) channel and triplet‐polaron quenching process prevails in the electroplex EML. It was also found that 3CT excitons behave as charge scattering sites. [ABSTRACT FROM AUTHOR]

Pure organic molecules based thermally activated delayed fluorescence (TADF) emitters have been successfully developed in recent years for their propitious application in highly efficient organic light emitting diodes (OLEDs). In the case of orange red emitters, the non‐radiative process is known to be a serious issue due to its lower lying singlet energy level. However, recent studies indicate that there are tremendous efforts put to develop efficient orange red TADF emitters. In addition, the external quantum efficiency (EQE) of heteroaromatic based orange red TADF OLEDs surpassed 30 %. Such heteroaromatic type emitters showed wide emission spectra; therefore, more attention is being paid to develop highly efficient orange red TADF emitters along with good color purity. Herein, the recent progress of orange red TADF emitters based on molecular structures, such as cyanobenzene, heteroaromatic, naphthalimide, and boron‐based acceptors, are reviewed. Further, our insight on these acceptors has been provided by their photophysical studies and device performances. Future perspectives of orange red TADF emitters for real practical applications are discussed. [ABSTRACT FROM AUTHOR]

In this study, two new donor-acceptor type thermally activated delayed fluorescence materials using carbazole as a donor and cyano substituted benzene as an acceptor were synthesized. Structural parameters, molecular orbital densities, photophysical characteristics were determined. By changing the substitution position of the cyano groups on the phenyl rings, the relationship between structure and properties was also be disclosed. These materials were tested as emissive species for the fabrication of organic light-emitting diodes. The blue devices showed EL emission peaks at 464 and 488 nm with the CIE coordinates of (0.17, 0.18) and (0.21, 0.31), respectively. [ABSTRACT FROM AUTHOR]

Organic light‐emitting materials exhibiting thermally activated delayed fluorescence (TADF) show great promise for improving display applications. Recently, intermolecular effects between emitting molecules have been given more attention, revealing strong solid‐state solvation or aggregation induced changes of sample performance. Implications of this on device performance are not yet fully covered. In this work, a thorough investigation of a novel TADF emitter, methyl 2,3,4,5,6‐penta(carbazol‐9‐yl)benzoate (5CzCO2Me), is provided. Steady‐state emission spectra reveal a luminescence redshift with increasing emitter concentration in a small molecule host. In all investigated concentrations, the emission profile remains the same; thus, the redshift is attributed to the solid‐state solvation effect. The highest photoluminescence quantum yield (PLQY) is achieved in the 20 wt% sample, reaching 66%. The best organic light‐emitting diode (OLED) in terms of current–voltage–luminance and external quantum efficiency (EQE) parameters is the device with 60 wt% emitter concentration, reaching maximal EQE values of 7.5%. It is shown that the emitter transports holes and that charge‐carrier recombination does not take place on the bandgap of the host, but rather, a mixed host–guest concentration‐dependent recombination is seen. The hole‐transporting properties of 5CzCO2Me allow for a new dimension in tuning the device performance by controlling the emitter concentration. [ABSTRACT FROM AUTHOR]

The increasing commercial applications of organic light‐emitting diodes (OLEDs) inspire great development of high‐efficiency emissive materials. Among of which, pure organic thermally activated delayed fluorescence (TADF) materials have been considered as the third‐generation OLED emissive materials, and huge attentions are paid to the exploration of molecular design strategies for achieving high efficiency TADF materials. Accordingly, the past years have witnessed a considerable rise in device performance of TADF‐mechanism based OLEDs. In this paper, we would like to provide a brief summary of our research work regarding to TADF materials as well as its applications in OLED including white OLEDs. [ABSTRACT FROM AUTHOR]

Recently, remarkable progress has been made in the field of organic light‐emitting diodes (OLEDs). Here, we show our recent results on multiscale charge transport simulation, thermally activated delayed fluorescence as emitters and sensitizers for OLEDs, and dynamic nuclear polarization‐NMR analysis. [ABSTRACT FROM AUTHOR]

Organic light‐emitting diodes (OLEDs) are a promising light‐emitting technology useful for various display applications1,2. Despite great progress in this field3−12, there is an ongoing challenge to realize high performance blue OLEDs with efficiency, good color purity, and device lifetime. Here, we report pure‐blue (CIEx,y color coordinates of [0.13, 0.16]) OLEDs with high‐efficiency (external quantum efficiency of 32 % at 1000 cd m−2), narrow‐emission (full‐width half maximum of 19 nm), and good stability (LT95 of 18 hours at an initial luminance of 1000 cd m−2). The design is based on a two‐unit stacked tandem hyperfluorescence (HF)‐OLED with an improved singlet‐excited energy transfer process from a sky‐blue TADF assistant dopant (AD) (HDT‐1) to a pure‐blue terminal emitter (TE) (v‐DABNA). Further, with sophisticated control of device architectures and materials combination, we report improved OLEDs. [ABSTRACT FROM AUTHOR]

Two blue thermally activated delayed fluorescence (TADF) emitters bearing di‐tert‐butyl carbazoles as the electron donor groups and pyrazine (DTCz‐Pz) or dipyrazine (DTCz‐Pz) as the electron acceptor are presented. The DFT calculations predict DTCz‐Pz and DTCz‐DPz to possess high S1 energies (3.19 eV and 3.08 eV, respectively), and relatively large ΔEST values (0.52 eV and 0.56 eV, respectively). The closely layered intermediate triplet states between S1 and T1, predicted by DFT calculations, are expected to facilitate the reverse intersystem crossing (RISC) and improve spin‐vibronic coupling efficiency between the excited states, even the relatively larger ΔESTs. The ΔESTs for DTCz‐Pz and DTCz‐DPz are 0.27 eV and 0.38 eV, and both molecules show high photoluminescence quantum yields (65 %, and 70 %, respectively). The decay lifetimes show temperature dependence in a PPT host, which is consistent with the fact that both molecules are TADF emitters in PPT. The OLEDs based on DTCz‐Pz exhibit deep blue emission with λEL of 460 nm and CIE of (0.15, 0.16). The maximum external quantum efficiency (EQEmax) reaches 11.6 %, with a maximum luminance (Lmax) of up to 6892 cd m−2, while the device based on DTCz‐DPz exhibits sky blue emission with λEL of 484 nm and CIE of (0.15, 0.30), an EQEmax of 7.2 %, and Lmax of 8802 cd m−2. [ABSTRACT FROM AUTHOR]

The carbazole of a model compound CPTBF was replaced by α‐ and β‐carboline to give donors α‐CPTBF and β‐CPTBF, respectively. The introduction of carboline leads the new donors to have deeper highest occupied molecular orbital (HOMO) energy levels. Different electron acceptors were tested, among them, a new acceptor, 3,4‐CN, was found to give exciplexes with efficient green emissions that are blue‐shifted as compared to that of model CPTBF:3,4‐CN system. The exciplex formations of α‐CPTBF:3,4‐CN and β‐CPTBF:3,4‐CN blends were verified with the significantly red‐shifted emissions different from those of constituent donor and acceptor together with the delayed fluorescent observed by time‐resolved PL decay experiments. The organic light‐emitting diode (OLED) devices with α‐CPTBF:3,4‐CN and β‐CPTBF:3,4‐CN blends as the emitting layer showed a maximum external quantum (EQE) of 7.57 and 7.34%, respectively, which is higher as compared to that (EQE = 6.87%) of the model device employing CPTBF:3,4‐CN. These results were attributed to the higher exciplex photoluminescence quantum yields due to the higher delay fluorescence components, deeper HOMO, and higher triplet energy of the carboline donors. In addition, the β‐CPTBF:3,4‐CN exciplex‐based OLED exhibited better efficiency roll‐off at higher luminesce due to more charge balance with less polaron formation, which was analyzed with time‐resolved EL. [ABSTRACT FROM AUTHOR]

To reduce cost and improve environmental sustainability, there continues to be an important need for the development of efficient organic light‐emitting diodes (OLEDs) that do not rely on heavy metal‐containing compounds. In particular, the efficiency of fluorescent near‐infrared (NIR) OLEDs continues to lag well‐behind that of their platinum‐containing counterparts. Low efficiencies in this spectral range mainly arise from the low quantum yields of fluorescent NIR emitters due to the energy gap law and inefficient harvesting of triplet excitons. In this paper, a thermally activated delayed fluorescent (TADF) material is used as the assistant dopant to demonstrate pure NIR‐emitting fluorescent OLEDs with an external quantum efficiency of up to 3.8%, with an electroluminescence maximum at 840 nm and a spectral full‐width at half‐maximum of < 40 nm. The efficiency is more than three times higher than that of the best previously reported fluorescent OLEDs in this spectral range and approaches that achievable with the best platinum‐containing phosphorescent emitters. [ABSTRACT FROM AUTHOR]

Doubly boron‐doped thermally activated delayed fluorescence (TADF) emitters based on a 9,10‐diboraanthracene (DBA) acceptor decorated with ortho‐donor groups (Cz2oDBA, 2; BuCz2oDBA, 3; DMAC2oDBA, 4) are prepared to realize high‐efficiency green‐to‐red organic light‐emitting diodes (OLEDs). X‐ray diffraction analyses of 2 and 4 reveal the symmetrical and highly twisted ortho‐donor–acceptor–donor (D‐A‐D) structure of the emitters. The twisted conformation leads to a very small energy splitting (ΔEST <0.08 eV) between the excited singlet and triplet states that gives rise to strong TADF, as supported by theoretical studies. Depending on the strength of the donor moieties, the emission color is fine‐tuned in the visible region from green (2) to yellow (3) to red (4). Carbazole‐containing 2 and 3 exhibit high photoluminescence quantum yields (PLQYs) approaching 100 %, whereas DMAC‐substituted 4 is moderately emissive (PLQY=44 %) in a doped host film. Highly efficient green‐to‐red TADF‐OLEDs are realized with the proposed ortho‐D‐A‐D compounds as emitters. The green and yellow OLEDs incorporating Cz2oDBA (2) and BuCz2oDBA (3) emitters exhibit high external quantum efficiencies (EQEs) of 26.6 % and 21.6 %, respectively. In particular, the green device shows an excellent power efficiency above 100 lm W−1. A red OLED fabricated with a DMAC2oDBA (4) emitter exhibits a maximum EQE of 10.1 % with an electroluminescence peak at 615 nm. [ABSTRACT FROM AUTHOR]

Widely investigated thermally activated delayed fluorescence (TADF) can be achieved by intramolecular and intermolecular charge transfer between an electron donor and electron acceptor which corresponds to a TADF material and exciplex, respectively. However, the development of efficient organic light‐emitting diodes (OLEDs) based on an exciplex lags far behind the development of those based on efficient TADF materials. In this work, a novel D‐A type electron donor TPAFPO was designed and synthesized. TPAFPO:PO‐T2T exhibits a small ΔEST of 79 meV and significant delayed emission, demonstrating TADF characteristics. OLEDs based on TPAFPO:PO‐T2T exhibit a low turn‐on voltage of 2.4 V and high an EQE value of 17.0%. Besides, NIR OLEDs utilizing TPAFPO:PO‐T2T as host exhibit a turn‐on voltage of 3.0 V and high EQE of 9.2% with a NIR emission peak at 690 nm. Furthermore, solution‐processed exciplex and NIR devices also can maintain high efficiencies of 15.1% and 8.1%, respectively. [ABSTRACT FROM AUTHOR]

The meta junction is proposed to realize efficient thermally activated delayed fluorescence (TADF) in donor–acceptor (D‐A) conjugated polymers. Based on triphenylamine as D and dicyanobenzene as A, as a proof of concept, a series of D‐A conjugated polymers has been developed by changing their connection sites. When the junction between D and A is tuned from para to meta, the singlet–triplet energy splitting (ΔEST) is found to be significantly decreased from 0.44 to 0.10 eV because of the increasing hole–electron separation. Unlike the para‐linked analogue with no TADF, consequently, the meta‐linked polymer shows a strong delayed fluorescence. Its corresponding solution‐processed organic light‐emitting diodes (OLEDs) achieve a promising external quantum efficiency (EQE) of 15.4 % (51.9 cd A−1, 50.9 lm W−1) and CIE coordinates of (0.34, 0.57). The results highlight the bright future of D‐A conjugated polymers used for TADF OLEDs. [ABSTRACT FROM AUTHOR]

Recent studies have demonstrated that in thermally activated delayed fluorescence (TADF) materials, efficient reverse intersystem crossing occurs from nonradiative triplet exited states to radiative singlet excited states due to a small singlet–triplet energy gap. This reverse intersystem crossing significantly influences exciton annihilation processes and external quantum efficiency roll‐off in TADF based organic light‐emitting diodes (OLEDs). In this work, a comprehensive exciton quenching model is developed for a TADF system to determine singlet–singlet, singlet–triplet, and triplet–triplet annihilation rate constants. A well‐known TADF molecule, 3‐(9,9‐dimethylacridin‐10(9H)‐yl)‐9H‐xanthen‐9‐one (ACRXTN), is studied under intensity‐dependent optical and electrical pulse excitation. The model shows singlet–singlet annihilation dominates under optically excited decays, whereas singlet–triplet annihilation and triplet–triplet annihilation have strong contribution in electroluminescence decays under electrical pulse excitation. Furthermore, the efficiency roll‐off characteristics of ACRXTN OLEDs at steady state is investigated through simulation. Finally, singlet and triplet diffusion length are calculated from annihilation rate constants. [ABSTRACT FROM AUTHOR]

To explore the correlation of the acceptor electron affinity and the molecular conformation to the thermally activated delayed fluorescence (TADF) feature, a series of d‐π‐A molecules were designed and synthesized with triazine (Trz) as the acceptor (A) and carbazole (Cz) or tert‐butylcarbazole (BuCz) as the donor (D). On the phenylene bridge between D and A, methyl or trifluoromethyl was incorporated close either to D or to A to tune the molecular conformation and the electron‐withdrawing ability of acceptor. Both the twist angles and the singlet and triplet energy difference (ΔEST) were observed strongly dependent on the type and position of the substituent on the π‐bridge. Only those molecules with trifluoromethyl locating close to the D side, namely TrzCz‐CF3 and TrzBuCz‐CF3, exhibit TADF feature, verifying that both sufficient electron affinity of the A unit and large dihedral angle between D and the π‐bridge are necessary to ensure the occurrence of TADF. The blue organic light‐emitting diodes fabricated with TrzCz‐CF3 and TrzBuCz‐CF3 achieved external quantum efficiencies of 9.40 % and 14.22 % with CIE coordinates of (0.19, 0.23) and (0.18, 0.29) respectively. This study provides practical design strategy for blue TADF materials particularly when planar and less crowded group is used as donor. [ABSTRACT FROM AUTHOR]